U.S. application Ser. No. 646,259, filed Jan. 2, 1976 by J. E. Pierson and S. D. Stookey, now U.S. Pat. No. 4,017,318, describes the making of photosensitive colored glasses or polychromatic glasses, as they have been variously called. Two fundamental methods for preparing such glasses are disclosed therein, each method involving a sequence of irradiation and heat treating steps. The glasses can be composed of a wide range of base compositions but each must contain silver, an alkali metal oxide which is preferably Na.sub.2 O, fluoride, and at least one halide selected from the group of chloride, bromide, and iodide. The glasses are irradiated with high energy or actinic radiations selected from the group of high velocity electrons, X-radiations, and ultra-violet radiations in the range of about 2800A-3500A. The heat treatments involve exposures to temperatures between about the transformation range of the glass up to about the softening point thereof. Where the actinic radiation is supplied as ultra-violet radiation, CeO.sub.2 is a required component of the glass composition.
In one of the methods described therein, the glass is initially exposed to high energy or actinic radiations. This exposure develops a latent image in the glass. The intensity and time of this exposure determine the final color which will be produced in the glass. Thereafter, the glass is subjected to a heat treatment which causes the precipitation of colloidal silver particles in situ to act as nuclei. Where a transparent final product is desired, the heat treatment will be undertaken only for so long as to effect the precipitation of colloidal silver nuclei and to cause the growth thereon of extremely small microcrystals of alkali metal fluoride-silver halide, e.g., NaF + (AgCl and/or AgBr and/or AgI). If an opal glass product is sought, the heat treatment will be continued for a sufficient length of time to not only promote the precipitation of colloidal silver nuclei, but also to effect the growth of said microcrystals on the silver nuclei to a size large enough to scatter light. The nucleated glass in then cooled -- conveniently to room temperature but, in any event, to a temperature at least 25.degree. C. below the strain point of the glass -- and again exposed to high energy or actinic radiations. This second exposure intensifies the color, the hue of which was previously determined via the first exposure. Finally, the glass is reheated to a temperature between about the transformation range and the softening point of the glass to produce the desired color in the glass. It has been theorized that submicroscopic particles of metallic silver are precipitated as discrete colloidal particles and/or deposited on the surface and/or within the alkali metal fluoride-silver halide microcrystals.
Although the mechanism of the color phenomenon is not undisputably known, the quantity of silver precipitated and the geometry thereof, as well as, perhaps, the refractive index of the crystals, are deemed to determine the color produced. However, since the colors can be achieved with very low silver contents and exhibit characteristics similar to interference colors, it was surmised that at least one of the three following circumstances is present: (1) discrete colloidal particles of silver less than about 200A in the smallest dimension; (2) metallic silver deposited within alkali fluoride-silver halide microcrystals, the silver-containing portion of the microcrystals being less than about 200A in the smallest dimension; and (3) metallic silver deposited upon the surface of said microcrystals, the silver-coated portion of the microcrystals being less than about 200A in the smallest dimension.
It was then observed that the heat treatment after each exposure to high energy or actinic radiation might consist of a series of heatings and coolings rather than a single treatment as delineated above. Such do not change the color developed but can improve color intensity.
That application also observed that the sequence of colors developed was dependent upon the flux of the initial exposure, i.e., the intensity and/or time of the exposure. Hence, the shortest initial exposure resulted in the development of a green color, followed by blue, violet, red, orange, and yellow as the exposure time and/or intensity was increased.
As has been observed above, the use of consecutive or interrupted heat treatments, either after the initial exposure to high energy or actinic radiation or after the second exposure thereto, can be helpful in intensifying the final color produced. Hence, although the mechanism involved is not fully understood, it appears that two or more treatments at temperatures between the transformation range and the softening point of the glass, separated by a cooling to below the transformation range, provide a more vivid color than a single heat treatment of equal or longer duration.
Furthermore, whereas that application discloses the utility of cooling the heat treated glass only to a temperature at least 25.degree. C. below the transformation range and then exposing it to high energy or actinic radiation, no working example of that embodiment of the invention is supplied. In all of the exemplary illustrations of the inventive processes, the exposures were conducted at ambient temperatures.